The present invention relates to an electromagnetic shock wave source in which the apparatus for generating the shock waves has a mechanically prestressed membrane made of an electrically conductive material, one side of which contacts an acoustic propagation medium, and the other side of which, is connected by way of an insulator with a flat coil.
Such shock wave sources are used for many different purposes. For example, in the medical field they are used for non-invasive crushing of concrements situated in a patient's body and non-invasive treatment of pathological tissue changes. In the former case, positive pressure pulses--that is, excess pressure pulses--are used. In addition, such shock wave sources can be used, for example, to test materials in order to act upon them by pressure pulses.
In such devices, the shock wave source is appropriately acoustically coupled with the object which is to be exposed to sonic waves, so that the pressure pulses generated in the acoustic propagation medium can be introduced therein. An important component of such a shock wave source is the part which generates the shock waves, which normally consists of an electrically conductive membrane and a flat coil situated opposite it. The shock waves are generated by connecting the flat coil to a high-voltage supply--for example, a capacitor which is charged to several kV. The discharge current flows through the flat coil, rapidly building up a magnetic field. The magnetic field, in turn, generates a current in the membrane, which is opposite to that of the flat coil, so that an opposing magnetic field is built up which moves the membrane abruptly away from the flat coil. The resulting shock wave is supplied to its intended destination (for example, a patient's body) via a corresponding acoustic propagation medium, such as degassed water.
In order to achieve a high efficiency (that is, the conversion of as much of the consumed electric energy as possible into acoustic shock wave energy), the membrane must rest against the flat coil as closely as possible, and must return to its starting position during the time period between two successive shock waves.
In conventional electromagnetic shock wave sources, it is therefore customary to evacuate the space between the coil and the membrane. To improve the electromagnetic coupling of the membrane with the coil, it is also customary as an alternative to apply an excess pressure to the propagation medium between the membrane and the patient, so that the membrane is pressed against the flat coil.
However, such a construction has certain disadvantages. Thus, particularly when no additional separating membrane is used, the excess pressure in the propagation medium can be used only in a low pressure range because the propagation medium (normally water in the case of all modern shock wave sources) is enclosed in a rubber-type double cushion which, in turn, can accommodate only a low excess pressure. In addition, to form and maintain the vacuum between the flat coil and the membrane, a vacuum pump and additional sealing measures are required. The excess pressure as well as the vacuum must be electrically controlled and monitored, which adds considerable servicing expenditures for the pumps generating the pressures.
German Patent Document DE C 4242131 describes an acoustic pressure pulse generator for generating acoustic pressure pulses in an acoustic propagation medium. The generator has a membrane that can be driven in a shock-like manner, adjoins the acoustic propagation medium and is mechanically prestressed so that it returns to its starting position after generating a pressure pulse. Before mounting, the membrane is curved toward the coil and is prestressed by pressing it along its edge against the shock exciting system, so that within its edge, the membrane rests against the shock exciting system, and is therefore subjected to compressive stress.
In this known pressure pulse generator, it must be ensured that, before a pressure pulse is generated, the membrane rests on the shock exciting system and, after a pressure pulse is generated, the membrane returns into its starting position. The membrane itself is a composite component consisting of a curved metallic disk which is mechanically connected (for example, by means of vulcanizing) to an edge part that can be clamped in place. In addition to the substantial technical expenditures required to manufacture such a composite component, there is also the significant disadvantage that a curved metallic disk can be placed flatly against the surface of a flat coil only by means of upsetting, axially acting forces without forming folds.
It is an object of the present invention to provide a shock wave source which has a particularly simple construction with a high efficiency at reasonable cost.
This object is achieved by the electromagnetic shock wave source according to the invention in which the electrically conductive membrane has a planar contour, and is prestressed by means of two structural parts which clamp the edge of the membrane and generate a force which acts in parallel to the surface of the membrane. One structural part has a recess into which the other structural part engages, while taking along the edge of the membrane.
The shock wave source according to the invention offers the advantages not only that the use of excess pressure in front of the membrane and/or of a vacuum between the membrane and the flat coil is unnecessary (thus eliminating all related expenditures for adjusting and monitoring the pressure, as well as for sealing), but also that a particularly simple construction is provided for the membrane and the flat coil generating the shock waves. The flat membrane requires no separate edge part to which it would have to be glued, so that the manufacturing is clearly less expensive.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.